A vulnerability that has existed in Android for the past four years can allow hackers to modify any legitimate and digitally signed application in order to transform it into a Trojan program that can be used to steal data or take control of the OS.

Researchers from San Francisco mobile security startup firm Bluebox Security found the flaw and plan to present it in greater detail at the Black Hat USA security conference in Las Vegas later this month.

The vulnerability stems from discrepancies in how Android apps are cryptographically verified, allowing an attacker to modify application packages (APKs) without breaking their cryptographic signatures.

When an application is installed and a sandbox is created for it, Android records the application's digital signature, said Bluebox Chief Technology Officer Jeff Forristal. All subsequent updates for that application need to match its signature in order to verify that they came from the same author, he said.

This is important for the Android security model because it ensures that sensitive data stored by one application in its sandbox can only be accessed by new versions of that application that are signed with the original author's key.

The vulnerability identified by the Bluebox researchers effectively allows attackers to add malicious code to already signed APKs without breaking their signatures.

The vulnerability has existed since at least Android 1.6, code named Donut, which means that it potentially affects any Android device released during the last four years, the Bluebox researchers said Wednesday in a blog post.

"Depending on the type of application, a hacker can exploit the vulnerability for anything from data theft to creation of a mobile botnet," they said.

The vulnerability can also be exploited to gain full system access if the attacker modifies and distributes an app originally developed by the device manufacturer that's signed with the platform key -- the key that manufacturers use to sign the device firmware.

"You can update system components if the update has the same signature as the platform," Forristal said. The malicious code would then gain access to everything -- all applications, data, accounts, passwords and networks. It would basically control the whole device, he said.

Attackers can use a variety of methods to distribute such Trojan apps, including sending them via email, uploading them to a third-party app store, hosting them on any website, copying them to the targeted devices via USB and more.

Some of these methods, especially the one involving third-party app stores, are already being used to distribute Android malware.

Using Google Play to distribute apps that have been modified to exploit this flaw is not possible because Google updated the app store's application entry process in order to block apps that contain this problem, Forristal said. The information received by Bluebox from Google also suggests that no existing apps from the app store have this problem, he said.

However, if an attacker tricks a user to manually install a malicious update for an app originally installed through Google Play, the app will be replaced and the new version will no longer interact with the app store. That's the case for all applications or new versions of applications, malicious or non-malicious, that are not installed through Google Play, Forristal said.

Google was notified of the vulnerability in February and the company shared the information with their partners, including the members of the Open Handset Alliance, at the beginning of March, Forristal said. It is now up to those partners to decide what their update release plans will be, he said.

Forristal confirmed that one third party device, the Samsung Galaxy S4, already has the fix, which indicates that some device manufacturers have already started releasing patches. Google has not released patches for its Nexus devices yet, but the company is working on them, he said.

Google declined to comment on the matter and the Open Handset Alliance did not respond to a request for comment.

The availability of firmware updates for this issue will differ across device models, manufacturers and mobile carriers.

Whether a combination of device manufacturers and carriers, which play an important role in the distribution of updates, coincide to believe that there is justification for a firmware update is extremely variable and depends on their business needs, Forristal said. "Ideally it would be great if everyone, everywhere, would release an update for a security problem, but the practical reality is that it doesn't quite work that way, he said."

The slow distribution of patches in the Android ecosystem has long been criticized by both security researchers and Android users. Mobile security firm Duo Security estimated last September, based on statistics gathered through its X-Ray Android vulnerability assessment app, that more than half of Android devices are vulnerable to at least one of the known Android security flaws.

Judging by Android's patch distribution history so far, the vulnerability found by the Bluebox researchers will probably linger on many devices for a long time, especially since it likely affects a lot of models that have reached end-of-life and are no longer supported.

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